1. Field of the Invention
The present invention relates to a mechanical seal used as a sealing device for a rotating shaft in a pump, a refrigerator, or the like, more particularly to a sliding part (ring) of a mechanical seal which is made of a silicon carbide sintered body.
2. Description of the Related Art
Materials used for sliding parts (e.g., a stationary ring or rotary ring) of mechanical seals include carbon materials such as bonded carbon bodies and resin-impregnated carbon bodies, cemented carbide bodies, silicon carbide sintered bodies, alumina sintered bodies, and silicon nitride sintered bodies. Increased use is being made of combinations of silicon carbide sintered body parts with other carbon material parts or with other silicon carbide sintered body parts, because silicon carbide sintered parts enable use at a higher PV limiting value (product of pressure applied to sliding surface and circumferential speed of rotary part). A high PV limit enables an increased performance and miniaturization of the sealing device and the apparatus using same. Further, combinations of two silicon carbide sliding parts are frequently used where there is a problem in particle wear due to handling of the slurry.
Silicon carbide sintered bodies have a high hardness, high wear resistance, and a crystal structure with few vitreous grain boundaries. Although silicon carbide sintered bodies are not self-lubricating as are carbon and hexagonal system boron nitride, silicon carbide sintered bodies have a superior smoothness, and therefore, have a small friction coefficient when sliding.
However, when a mirror surface of a silicon carbide sliding part comes into contact with a mirror surface of another silicon carbide sliding part, problems such as abnormal noise (squeaking) and linking at the startup (initial) period easily occur. To solve these problems, an improvement of the dimensional accuracy of the parts, the accuracy of the device and the accuracy of mounting have been proposed. Hard material contacts, however, have a poor familiarity, and thus the problems have not been solved.
When a mirror surface of a silicon carbide sliding part comes into contact with another carbon material sliding part having a self-lubricity, no abnormal noise and linking occur, but carbon blisters appear. This phenomenon starts with a generation of blisters on the sliding surface of the carbon material part, advances to formation of microcracks, and finishes with worm defects (e.g., chipping). Since the defects cause liquid leakage (spill), carbon blisters are a serious defect in mechanical seals. Carbon blisters occur in combinations of carbon material parts and parts of other materials including silicon carbide. It is considered that the frictional heat generated at the startup period causes an alternate expansion and contraction of the carbon material part surface, and that this causes fatigue, and simultaneously, thermal stress failure at the surface. Other factors behind carbon blisters are considered to be (a) thermal decomposition of the impregnating oil in the carbon material part and (b) the explosive reaction of oil held in pores of the carbon material part caused by the frictional heat. Combinations of carbon material sliding parts and other silicon carbide sliding parts are often used in applications involving a high sliding surface pressure. In such cases, the problem of carbon blisters due to the frictional heat at the starting period becomes particularly serious.
The following measures have been proposed to deal with the problem of blisters: (a) increase of strength of the carbon material; (b) improvement of mounting accuracy of two sliding rings to bring them into contact more uniformly; (c) adoption of a double seal; (d) flushing by a low viscosity fluid; and (e) steam heating to raise the sliding surface temperature and lower the viscosity of the sealed liquid. These measures, however, are not sufficient.
In any case, the true solution to the problems is to lower the frictional heat at the start up period, and to lower the frictional heat, it has been proposed to use a reaction-sintered silicon carbide including residual metallic silicon, and to impregnate pores of a porous silicon carbide sintered body with a solid lubricant, as disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 62-148384, 62-270481, and 63-79775.
Furthermore, a silicon nitride sintered body (ring) for a mechanical seal free from abnormal noise (squeaking) is proposed in Japanese Unexamined Patent Publication No. 62-176970. In this case, one of a pair of sliding parts (rings) is made of a silicon nitride sintered body having either a porosity of from 8% to less than 13% and an average pore diameter of from 50 to 500 .mu.m or a porosity of 13% or more and an average pore diameter of from 25 to 500 .mu.m.
A silicon carbide sintered body produced by the reaction sintering method and containing metallic silicon has less drag and abnormal noise than a silicon carbide sintered body produced by a pressureless (atmospheric pressure) sintering method. The former silicon carbide sintered body, however, has an inferior corrosion resistance, and cannot be widely used. Also, to impregnate a porous silicon carbide sintered body with solid or liquid lubricant such as molybdenum disulfide, graphite, boron nitride, or fluorocarbon oil, it is necessary to repeat vacuum impregnation several times, which raises the production cost. Small pores having a pore diameter of less than 50 .mu.m cannot be impregnated within a short time, whereas large pores having a pore diameter of 50 .mu.m or more lower the strength and wear resistance of the sintered body.